Digital systems normally operate in synchronism with sequential ticks of one or more clock signals. The ticks of a clock signal, which are defined by voltage transitions representing its leading or trailing edges, control processing circuitry that decodes, processes and stores data signals in response to control signals, in a sequence of processing cycles. The rate at which a digital system can process data is determined, in part, by the maximum amount of time required for the data and control signals to propagate through the processing circuitry in one processing cycle.
Typical digital circuitry is comprised of a number of integrated circuit chips which contain much of the processing circuitry. Many of the chips have one or more clock terminals, each terminal receiving a clock signal that controls the circuitry on the chip. Typically, a clock terminal on a chip is connected to a clock buffer circuit that drives a network of lines on the chip used to distribute the clock signal to the processing circuitry thereon.
As a result of various delays inherent in propagating a clock signal through a clock buffer circuit, the edges of the clock signal are delayed, such that the edges of the clock signal at the output of the clock buffer circuit do not occur in precise synchronism with the edges of the clock signal at the clock terminal. While this might not be a major problem if the propagation delay were uniform in all of the chips that are used in a system, in fact the propagation delay from chip to chip may vary unpredictably by a substantial amount as a result of variations in manufacturing from chip to chip. To accommodate such unpredictable variations, the clock period has to be further increased (otherwise stated, the time between edges has to be further increased), which reduces the rate at which processing can be accomplished by the system. Alternately, chips can be tested and selected for use in a system if they meet selected criteria in the uniformity of the propagation delay exhibited thereby, but that would result in a likely increase in the cost of the system.